Considerable effort has been expended in recent years in the development of improved heterogeneous catalysts for chemical reactions, particularly for the partial or complete oxidation of volatile carbon compounds in air and for the reduction of nitrogen oxides to nitrogen by hydrogen, carbon monoxide, and other carbon compounds. Such efforts have been directed not only toward the development of more effective catalysts for use in the manufacture of organic chemicals and for the reduction of atmospheric pollution by industrial processes involving the manufacture and use of nitric acid, but have also been directed toward the reduction of atmospheric pollution by exhaust gases from internal combustion engines.
Among the catalytic compositions which have been proposed for reducing the concentration of nitrogen oxides in off gases for nitric acid plants and exhaust gases of internal combustion engines are such platinum metals as platinum, palladium, rhodium, and ruthenium and the oxides of such metals of the first transition series of the periodic table as iron, cobalt, and nickel and of such rare earth metals as lanthanum, neodymium, and praseodymium.
Many materials have been suggested as catalysts for the oxidation of carbon monoxide, hydrocarbons and partial oxidation of products of hydrocarbons in the exhaust gases of internal combustion engines, including the oxides, cerates, chromates, chromites, manganates, manganites, molybdates, tungstates, carbonates, stannates, ferrites, and vanadates of such metals as iron, cobalt, nickel, zinc, palladium, platinum, ruthenium, rhodium, manganese, chromium, copper, cadmium, silver, calcium, barium, mercury, tin, lead, molybdenum, tungsten, and the rare earths and mixtures of these compounds and such precious metals as ruthenium, rhodium, palladium and platinum.
Also among the catalysts proposed for the reduction of nitrogen oxide, the oxidation of carbon monoxide and hydrocarbons, and other reactions involved in the purification of automotive exhaust gases are a group of metal oxides of the perovskite crystal type. For example, lanthanum cobaltite, neodymium cobaltite, dysprosium cobaltite, and a similar cobaltite containing a mixture of ions of rare earth metals have been shown to be effective heterogeneous catalysts for the hydrogenation and hydrogenolysis of cis-2-butene and these materials and similar perovskite metal oxides doped with metal ions having other valences (e.g., Sr.sub.0.2 La.sub.0.8 CoO.sub.3) have been considered for use as automotive exhaust oxidation catalysts.
While the rare earth cobaltites and other perovskite compositions have advantages over the earlier catalytic materials, there is still a need for catalytic compositions which optimize catalytic performance. It is reported that the catalytic activity of the platinum metals in oxidation processes is greatly reduced by a long-time exposure to high temperatures, apparently because of changes in particle size and crystal structure or because of the formation of volatile oxides. Other proposed catalysts are effective only at high temperatures that require catalyst supports and enclosures made of materials which are scarce and difficult to fabricate. It has also been reported that some of the proposed catalysts for the reduction of nitrogen oxides, such as platinum and palladium catalysts, promote the formation of undesirably large amounts of ammonia instead of nitrogen from nitrogen oxides when the reducing agent is hydrogen. Similarly, some catalysts promote the formation of undesirably large amounts of intermediate oxidation products in the oxidation of hydrocarbons instead of promoting complete oxidation to carbon dioxide and water. Other catalysts, including the platinum metals and some of the transition and rare earth metal oxides, are reported to lose their catalytic activity upon exposure to alternately oxidizing and reducing environments such as can be produced by industrial processes and internal combustion engines operating under frequently changing conditions. Still other proposed catalysts have reduced catalytic activity after exposure to normally nonreactive components of gas mixtures. For example, the transition and rare earth metal oxides are reported to have reduced activity as catalysts for the oxidation of carbon monoxide and hydrocarbons in the presence of water and the platinum metal catalysts lose their catalytic activity upon exposure to internal combustion engine exhaust gases containing compounds of lead, sulfur, phosphorus, chlorine and other materials derived from additives conventionally employed in automotive fuels and lubricants.
Thus there is a need for catalysts which are low in cost, selective in promoting desired oxidation and/or reduction reactions at relatively low temperatures, active for long periods at the temperatures involved and in the presence of the materials incidental to these reactions, simple to prepare in suitable forms having high catalytic activity, and active at relatively low surface areas per unit weight of catalytic material.